Bromus tectorum, a non-native annual grass, now dominates millions of hectares of low and mid elevation landscapes in the western United States. Where this occurs, native plant and animal diversity is reduced and sometimes extirpated, fire frequencies are increased, community productivity is decreased, and soil biota and nutrient cycles are altered. As almost all efforts to contain or eliminate Bromus have been unsuccessful, attention has turned to ways that invasions might be avoided or how invaded areas might be restored.
This project explored the following questions: (1) How can soil characteristics that may have influenced Bromus invasion be accurately assessed? (2) What site factors confer resistance to invasion by Bromus? Can these factors also predict areas susceptible to invasion on a landscape and regional scale? (3) Can soil factors that confer resistance be used to suppress Bromus while not affecting the germination or success of native plants? and (4) Once Bromus invades, how does it affect native communities and soil nutrient cycles in the absence of other disturbances? Do these alterations affect the ability of the site to support natives?
Using landscape assessments, vegetation and soil chemistry “fingerprints” of invaded and non-invaded areas of Bromus were established to determine habitats vulnerable to invasion. Greenhouse experiments were conducted for Bromus, Hilaria (a native grass), and a Bromus/Hilaria mix using a variety of nutrient amendments, and resin products were used to measure plant-available nutrients. Promising amendments that favored Hilaria and inhibited Bromus were further investigated in the field. Additionally, the role of invaded versus non-invaded soils and microenvironments was examined to better understand Bromus invasions.
Results indicate that soil chemistry plays a major role in determining whether or not a site will be invaded and that microhabitat and herbivory are less important. A predictive model was developed to forecast future invasions in the western United States. Bromus also was found to be very salt-sensitive; whereas, native grasses are salt-tolerant. There is evidence, however, that the effect of salt-based amendments to suppress Bromus change with precipitation regimes as well as over time. Once Bromus invades, it has differential effects on native communities depending on species present prior to invasion. Restoration of invaded grasslands appears to be a reachable management goal, but may require restriction of other disturbances. Changes in nutrient availability are unlikely to favor Bromus over natives during restoration efforts; however, increased nitrogen cycling rates will likely decrease soil nitrogen over long time periods (>100 years). Although Bromus alters nutrients slightly, it dramatically alters both the abundance and species composition of soil food webs. Given the ability of these soils to support growth of Hilaria, managers likely do not need to manipulate soil food webs or soil chemistry to successfully restore invaded areas.
This project will aid land managers in predicting areas susceptible to invasion by exotic annual grasses and will facilitate efforts to suppress these grasses without affecting native plants and to restore invaded grasslands. Understanding how exotic annual grass invasion changes natural ecosystem processes such as nutrient availability, water availability, and soil microbial systems and how these changes affect reestablishment of native plants further will enhance restoration efforts. (Project Completed – 2004)